RF spectrum is a sought-after resource that must be shared. There are increasingly more users wanting a piece of that spectrum. To make matters worse, users want to pass ever-increasing amounts of information over RF channels.

Information conveyed using traditional analog modulation schemes, such as AM or FM, requires relatively simple transmitters and receivers. However, with these modulation types, there are fixed rules relating to the amount of information that can be conveyed within a given bandwidth. The knock-on effect is that only so many users can be “fitted in.”

On the other hand, if digital modulation methods that require more complex transmitters and receivers were used, more information could be transmitted in that same amount of spectrum. Effectively, digital radio gives users greater spectral efficiency. This is the fundamental trade-off.

Industry Trends

During the past 20 years, a major transition has taken place from traditional analog modulation schemes such as AM and FM to new digital modulation techniques such as continuous four frequency modulation (C4FM), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM). A further level of complexity has been introduced with multiplexing schemes such as TDMA, CDMA and FDMA. Traditionally, only one conversation could take place on any one block of spectrum (channel) at a time, however, these multiplexing schemes make it possible to simultaneously maintain several separate conversations within the same channel.

Received Audio Quality

With analog communications, the reconstructed audio waveform at the receiver is derived from instantaneous amplitude (AM) or frequency (FM) of the received signal. At weak received signal levels, any noise on the received signal is replicated on the recovered audio and heard by the user.

With digital radio, the analog information (voice) is digitized into a stream of ones and zeros, filtered and modulated onto an RF carrier where it appears as prescribed changes (symbols) to the amplitude, frequency or phase of the transmitted RF signal. At the receiving end, these changes are decoded, and because there are only a defined number of possible symbols, accurate decoding can still be achieved in the presence of considerable noise. This means the original analog information can be almost perfectly reconstructed, even when the received RF signal was very weak.

What is important to the end user is the received audio clarity. In the case of analog FM, audio clarity is proportional to the signal to noise and distortion (SINAD) of the demodulated audio; in the case of digital radio, it is proportional to bit error rate (BER).

Having understood that, let’s compare the SINAD achieved by narrowband analog FM with the BER of digital C4FM with decreasing received signal level as illustrated in the image above. For narrowband analog FM, a demodulated SINAD of about 12 dB roughly corresponds to speech that is understandable with slight effort. For digital C4FM, similar audio clarity occurs when the BER is about 5 percent. So, we can see that the minimum signal level before speech becomes unintelligible in both cases, around -121 dBm.

However, if we compare performance at a received signal level of -115 dBm, the SINAD for narrowband FM is about 22 dB. At these SINAD levels, speech is understandable, but the noise is clearly audible too. For C4FM, the speech is still virtually error-free and clear.

In practice, the interference-free environment for which these measurements were made is a figment of our imagination. But with real-life propagation effects like multipath and fading, the basic premise holds true — digitally modulated systems deliver better audio quality at low received signal levels.

In summary there are three main motivators for moving to digital radio:

1. Efficient use of spectrum space.

2. Increased amounts of information that can be passed down a single channel.

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